High temperature gas cooled reactors (HTGR) hold significant promise for highly efficient and inherently safe production of electrical power. HTGR designs typically feature fuel that is isolated within layers of graphite moderator, and is constructed with multiple physical barriers to minimize the potential for fission product release. Also, the negative temperature feedback reactivity characteristics of HTGR fuel results in automatic shutdown of the nuclear fission chain reaction when temperatures exceed safe levels. These inherent characteristics of HTGRs enable the design of inherently safe reactors that rely on minimal emergency core cooling systems.
One type of HTGR is known as a “pebble bed” reactor. In this HTGR design, fissionable fuel is encapsulated within multilayered ceramic particles that are themselves encapsulated within multi-layered graphite spheres, referred to as “pebbles.” The spherical nature of the fuel pebbles enables gas to flow between the spheres for extracting heat from the reactor, while providing a core structure that is self assembling. Fuel pebbles are simply loaded into a cylindrically shaped core region that is formed by loosely packed graphite blocks that provide the structural support for the pebbles to remain in a randomly packed cylindrical shape. During refueling spent fuel can be removed simply by pebble unloading device which removes a single pebble at a time before feeding the spent fuel pebble to a spent fuel container using the force of gravity only. Fresh fuel pebbles are loaded into the top of the core barrel also using a gravity feed system.